Plug valves are very well known and are in common use, but are subject to a number of disabilities. Firstly, the shape of the plug within the socket is likely to vary with time, and accordingly leakage can take place. Secondly, if the plugs are inserted with sufficient interference into the respective sockets to minimise leakage, they tend to become difficult to rotate. Thirdly, quite frequently the plugs are retained by means of nuts or other threaded members which project through the small diameter end of the body, and this renders removal of the plug difficult if there is no access provided for the underside of the body.